Sugar composition not only affects fruit flavor but is also an important determinant of fruit taste and consumer preference. In this study, changes in the sugar content and sugar-metabolizing enzymes were investigated from different sections of various fruit development phases of ‘Hongshuijing’ pitaya (Hylocereus monacanthus). Genes related to sugar metabolism were also screened by transcriptome analyses. The results indicated that glucose was the major sugar in mature pitaya fruit, and was mainly regulated by vacuolar acid invertase (VAI) and sucrose synthase (SS) (degradative direction). Sugar accumulation varied in pulp between different sections of the pitaya fruit. VAI, neutral invertase (NI) and SS (degradative direction) are crucial enzymes for sugar accumulation in pitaya. The expression of 17 genes related to sucrose metabolism obtained from seven databases [NCBI non-redundant protein database (Nr), NCBI non-redundant nucleotide sequence database (Nt), EuKaryotic Orthologous Groups (KOG), The Protein Families (Pfam), Kyoto Encyclopedia of Genes and Genomes (KEGG), Swiss-prot, and Gene Ontology (GO)] were analyzed in different pitaya pulp sections. HpVAI1 had the highest relative expression level on the 29th day after pollination (DAP). Positive correlations were found between HpVAI1 expression and VAI activity; HpNI4 and NI activity; HpSS2, HpSS5, and SS activity (synthetic direction), indicating that HpVAI1, HpNI4, and HpSS2 and HpSS5 were involved in the regulation of VAI, NI, and SS (synthetic direction), respectively. HpVAI1 and HpNI4 regulated sucrose degradation and the accumulation of glucose and fructose, while HpSS2 and HpSS5 regulated sucrose synthesis. These results suggest that HpVAI1 plays a key role in sugar metabolism during fruit development of ‘Hongshuijing’ pitaya. The results of this study provide new information about sugar metabolism in pitaya fruit that could help improve fruit quality and the breeding of new cultivars.

糖分不仅影响水果风味，而且是水果口味和消费者偏好的重要决定因素。本研究从‘红水井’火龙果（ Hylocereus monacanthus）不同果实发育阶段的不同部位研究了糖含量和糖代谢酶的变化。）。与糖代谢相关的基因也通过转录组分析进行筛选。结果表明，成熟火龙果中葡萄糖是主要糖，主要受液泡酸转化酶（VAI）和蔗糖合酶（SS）（降解方向）调控。火龙果不同部分的果肉中的糖积累不同。VAI、中性转化酶（NI）和SS（降解方向）是火龙果糖分积累的关键酶。从 7 个数据库 [NCBI 非冗余蛋白质数据库 (Nr)、NCBI 非冗余核苷酸序列数据库 (Nt)、EuKaryotic Orthologous Groups (KOG)、The Protein Families (Pfam)、Kyoto 获得与蔗糖代谢相关的 17 个基因的表达在不同的火龙果果肉切片中分析了基因和基因组百科全书 (KEGG)、Swiss-prot 和基因本体论 (GO)]。HpVAI1在授粉后第29天（DAP）的相对表达水平最高。HpVAI1表达与VAI活性呈正相关；HpNI4和 NI 活性；HpSS2、HpSS5、SS活性（合成方向），说明HpVAI1、HpNI4、HpSS2、HpSS5分别参与VAI、NI、SS（合成方向）的调控。HpVAI1和HpNI4调节蔗糖降解以及葡萄糖和果糖的积累，而HpSS2和HpSS5调节蔗糖合成。这些结果表明HpVAI1对‘红水井’火龙果果实发育过程中的糖代谢起着关键作用。这项研究的结果提供了有关火龙果果实糖代谢的新信息，有助于提高果实品质和培育新品种。